Depends on your environment. When writing C/C++ code, I rely on the compiler to find many problems. Once a module compiles cleanly, I link with tests to verify and then the app. When using Python and Javascript, I just repeatedly run the tests and program. Of course, I do sort of dry run the function / method / class as I write it. I'm probably in the minority, but I do write copious comments to remind myself in the future why I did things the way I did.